Process for converting methane into ethane

ABSTRACT

The invention relates to a process for producing ethane comprising contacting methane with a metal catalyst selected from metal hydrides, metal organic compounds and mixtures thereof. It also relates to a process for the conversion of methane to carbon-containing products comprising contacting methane with a metal catalyst comprising at least one metal, Me, chosen from the lanthanides, the actinides and the metals from Groups 2 to 12 of the Periodic Table of the Elements, so as to produce ethane in a proportion of at least 65%, especially at least 98% or 99% by weight with respect to carbon-containing products formed in the process. The process can be a single-step process, preferably carried out under conditions involving a non-oxidative catalytic coupling of methane, in particular under operating conditions maintained substantially constant, preferably continuously, during the ethane production, e.g. at a temperature ranging from −30° C. to +80° C., preferably from 20° C. to 500° C., under a total absolute pressure ranging from 10 −3  to 100 MPa, preferably from 0.1 to 50 MPa. The metal catalyst may be chosen from metal catalysts supported on and preferably grafted to a solid support. One of the main advantages of the present invention is to produce ethane with a very high selectivity.

The present invention relates to a process for producing ethane frommethane and to a process for the conversion of methane to ethane.

Alkanes and in particular methane are products which are generallydifficult to employ in reactions because of their high chemical inertia,and are used essentially as fuels and energetic materials. Furthermore,methane, which is the main constituent of natural gas, is one of themost widespread sources of hydrocarbons in the world.

There are two main routes for converting methane: an indirect routeinvolves the intermediacy of a mixture of carbon monoxide and hydrogen,also known as “synthesis gas”, and makes it possible to convert methaneto liquid fuels or other chemicals, and a direct route converts methaneto methanol or to hydrocarbons, in particular to C₂ hydrocarbons. Mostindustrial processes for the conversion of methane use the indirectroute and thus convert methane to “synthesis gas” by steam reforming andsubsequently synthesize methanol or petrols using “synthesis gas” asintermediate. However, these processes require a great deal of energyand very high temperatures ranging beyond 800° C.

To avoid these difficulties, numerous studies have been carried out forthe purpose of developing processes for the conversion of methane by thedirect route. Mention may be made, among these processes, of oxidativecoupling, thermal coupling, plasma coupling and non-oxidative catalyticcoupling.

In particular, the oxidative coupling of methane consists in convertingmethane directly to ethane and to ethylene in the presence of oxygen andof a catalyst, and in then converting these hydrocarbons to liquidhydrocarbon fuels, such as petrols. However, the process by oxidativecoupling generally results in the formation of relatively large amountsof by-products, such as carbon monoxide and carbon dioxide, and has tobe carried out at high temperatures, in particular greater than 600° C.

Thermal coupling makes it possible to convert methane directly to C₂hydrocarbons at very high temperature. However, in addition to ethane,large amounts of ethylene and acetylene are formed, which products oftenconstitute the major products formed by the reaction. In addition, thethermal coupling process is carried out at extremely high temperatures,generally of greater than 1200° C.

Plasma coupling consists of activating methane to methyl radicals byvirtue of the very high energy provided by the plasma and in forming inparticular ethane, propane, ethylene, acetylene and even small amountsof C₄ hydrocarbons. Although this process can exhibit a high selectivityfor ethane under certain conditions, in particular in the presence ofglass or alumina beads (Korean J. Chem. Eng., 18(2), 196-201 (2001)), itrequires the use of high energy and does not appear to be veryattractive from the viewpoint of industrial application.

It is known (J. Phys. Chem. A, Vol. 103, No. 22, 1999, 4332-4340) tocarry out a catalytic oligomerization of methane by heating undermicrowaves in the presence of a catalyst chosen from nickel or ironpowders or from activated carbons. By virtue of the energy of themicrowaves as source of activation, this process makes it possible toform a mixture of C₂ to C₈ hydrocarbons, in particular C₂ to C₆hydrocarbons, for example a mixture of ethane, ethylene, acetylene andbenzene, and also carbon monoxide and carbon dioxide. The formation ofcarbon oxides appears to show that this process is an oxidative couplingmethod. In addition, this process is not sufficiently selective to formsolely light alkanes and in particular ethane.

It is also known (Chem. Commun., 1999, 943-944) to carry out, at 450°C., selective activation of methane to alkenes in the presence of ahydrogen accumulation system comprising titanium and 0.4% by weight ofnickel. Methane is converted to C₂ to C₄ alkenes and alkanes, thealkenes and in particular ethylene constituting the major productsformed. However, the activation of methane carried out under theseconditions is not sufficiently selective to form essentially lightalkanes and in particular ethane.

United States Patent U.S. Pat. No. 5,414,176 discloses a process forconverting methane to higher hydrocarbons, in particular to C₂ to C₇hydrocarbons. The process successively comprises bringing a gas streamconsisting essentially of methane into contact with a catalystcomprising a transition metal dispersed over a support based onrefractory oxide, then bringing the catalyst into contact with a streamof hydrogen, so as to form a gas mixture of higher hydrocarbons and ofhydrogen, subsequently recovering the gas mixture, and separating thehigher hydrocarbons from the hydrogen. However, the conversion ofmethane is not very selective, since it results in the formation of amixture of hydrocarbons ranging from C₂ to C₇. Furthermore, the processis relatively complex, since it comprises a sequence of several stages,in particular two successive contacting operations of the catalyst.German Patent Application DE 31 16 409 discloses a process for producinghigher hydrocarbons, in particular for producing C₂ hydrocarbons(essentially ethane, ethylene and acetylene) from methane. The processcomprises (i) a first stage for a dissociative chemisorption of methaneon a catalyst surface (e.g. a platinum catalyst) at a temperature of 180to 300° C., (ii) a second stage for cooling the chemisorbed intermediateproducts at a temperature of 120 to 150° C. so as to form higherhydrocarbons by a C—C recombination, and (iii) a third stage fordesorbing the higher hydrocarbons with a stream of hydrogen. The GermanPatent Application is silent about the ethane production compared withthe formation of the other higher hydrocarbons. In addition, itdiscloses a multi-stage process carried out at different temperaturesand involving the use of hydrogen in the last stage for producing thehigher hydrocarbons.

The present invention relates to a process for producing ethane frommethane, in particular to a process for converting methane essentiallyinto ethane, the process being advantageously carried out with a veryhigh selectivity by weight for ethane with respect to carbon-containingproducts formed. It is considered that the selectivity by weight forethane is generally at least 65%, preferably at least 70%, in particularat least 80%, especially at least 90%, and more especially at least 95%.The term “selectivity by weight for ethane” is generally understood tomean the part by weight of ethane formed per 100 parts by weight ofcarbon-containing products formed in the process. In particular, it isnoted that ethane can thus be formed directly with a very high degree ofpurity, for example with a selectivity by weight of at least 98% or evenof at least 99%.

The process of present invention can, in addition, advantageously becarried out without forming detectable amounts of carbon-containingproducts other than alkanes, for example of alkenes (e.g. ethylene), ofalkynes (e.g. acetylene), of aromatic compounds (e.g. benzene), ofcarbon monoxide and/or of carbon dioxide.

Furthermore, the process of the present invention can be advantageouslycarried out under conditions involving a non-oxidative catalyticcoupling of methane. Preferably, it is a single-stage process, inparticular carried out under operating conditions maintainedsubstantially constant, preferably continuously, during the ethaneproduction.

The Periodic Table of the Elements mentioned below is that proposed bythe IUPAC in 1991 and which is found, for example, in “CRC Handbook ofChemistry and Physics”, 76th Edition (1995-1996), by David R. Lide,published by CRC Press Inc. (USA).

A first subject-matter of the invention is a process for producingethane, characterized in that it comprises bringing methane into contactwith a metal catalyst chosen from metal hydrides, metal organic compounsand mixtures thereof. The metal catalyst preferably comprises at leastone metal, Me, chosen from the lanthanides, the actinides and the metalsfrom Groups 2 to 12, preferably 3 to 12, of the Periodic Table of theElements.

A second subject-matter of the invention is a process for the conversionof methane to carbon-containing products, characterized in that methaneis brought into contact with a metal catalyst comprising at least onemetal, Me, chosen from the lanthanides, the actinides and the metalsfrom Groups 2 to 12, preferably 3 to 12, of the Periodic Table of theElements, so as to produce ethane in a proportion of at least 65% byweight with respect to carbon-containing products formed in the process.

In the present description, the process of the invention generally meansthe process for producing ethane as well as the process for theconversion of methane.

In the process of the invention, methane reacts essentially with itself.This is generally known as the methane coupling reaction oralternatively methane homologation reaction. The reaction resultsessentially from bringing methane into contact with a metal catalyst andgenerally leads to the formation, in particular by a reversiblereaction, of ethane and hydrogen, especially according to the followingequation:2CH₄→C₂H₆+H₂  (1)

The process of the invention can be carried out by bringing methane intocontact with a metal catalyst under a total absolute pressure rangingfrom 10⁻³ to 100 MPa, preferably from 0.1 to 50 MPa, in particular from0.1 to 30 MPa or from 0.1 to 20 MPa, especially from 0.1 to 10 MPa.

The process of the invention can be also carried out at a temperatureranging from −30 to +800° C., preferably from 0 to 600° C., inparticular from 20 to 500° C. and especially from 50 to 450° C., forexample from 50 to 400° C. or from 50 to 350° C. The most preferredrange of the temperature is from 200 to 600° C., and especially from 250to 500° C.

The process of the invention can be carried out in various ways, forexample by adding the methane to the metal catalyst, or by adding themetal catalyst to the methane, or by simultaneously mixing the methaneand the metal catalyst.

Generally, the methane used in the present invention constitutesessentially the only initial alkane used in the conversion. However, theprocess of the invention can be carried out by contacting methane withthe metal catalyst in the presence one or more other initial alkane(s),such as those present in natural gas. The other optional initialalkane(s) can be chosen from C₂ to C₃₀ alkanes, preferably C₂ to C₂₀alkanes, in particular C₂ to C₁₂ or C₂ to C₁₀ alkanes, especially frompropane, n-butane, isobutane, n-pentane, isopentane, n-hexane,isohexane, n-beptane, isoheptane, n-octane and isooctane, and preferablyfrom propane and n-butane. Under certain conditions, it has been noticedthat the other initial alkane(s) can react by hydrogenolysis with thehydrogen produced by the methane coupling reaction and can thusfavourably shift the methane coupling reaction towards the formation ofethane. Such conditions can exist in particular when the process iscarried out in the presence of a hydrogenolysis catalyst. This isparticularly advantageous when the other optional initial alkane(s) arechosen from propane and n-butane, which, by hydrogenolysis, can producein particular ethane.

The other optional initial alkane(s) can preferably be present with themethane in relatively low proportions, so that the ethane is produced inparticular in a proportion of at least 65% by weight with respect tocarbon-containing products formed (or in one of the other proportionsmentioned above), that is to say with a high selectivity by weight forethane, such as one of those mentioned above. Thus, according to theinvention, methane can be used substantially as the only initial alkane,so that other initial alkane(s) can be used in an amount of less than10⁻⁵ mol, preferably than 5×10⁻⁶ mol, in particular than 10⁻⁶ mol, permole of methane

When the process of the invention is carried out in particularbatchwise, the metal catalyst can be brought into contact with themethane in a molar ratio of the methane to the metal, Me, of the metalcatalyst extending over a wide range, for example from 10:1 to 10⁵:1,preferably from 50:1 to 10⁴:1, in particular from 50:1 to 10³:1.

The process of the invention can be carried out in the presence of oneor more inert agents, in particular liquid or gaseous inert agents,especially in the presence of one or more inert gases, such as nitrogen,helium or argon.

The process of the invention can be carried out batchwise or,preferably, continuously. It can be carried out in gas phase, inparticular in a fluidized bed reactor and/or a reactor with amechanically stirred bed, or in a stationary bed reactor or circulatingbed reactor, in which reactor the bed can be formed essentially with themetal catalyst in a solid form, preferably of the metal catalystsupported on and grafted to a solid support, such as describedsubsequently. It is preferable to carry out the process of the inventioncontinuously and in gas phase, in particular in which methane isintroduced continuously into a reaction zone comprising the metalcatalyst, so as to form a gas mixture comprising ethane, the gas mixtureis continuously withdrawn from the reaction zone, the ethane produced isseparated, at least partially and continuously, in the withdrawn gasmixture, from unreacted methane and optionally hydrogen formed, theethane is thus recovered and, preferably, unreacted methane is returnedto the reaction zone.

The process of the invention is advantageously carried out with a metalcatalyst comprising at least one metal, Me, chosen from the lanthanides,the actinides and the metals from Groups 2 to 12, preferably 3 to 12, ofthe Periodic Table of the Elements. In particular, the process iscarried out in the presence of a metal catalyst selected amongst metalcatalysts suitable for converting methane into ethane with theabove-mentioned selectivity for ethane. More particularly, the metalcatalyst can be chosen from metal catalysts supported on and preferablygrafted to a solid support. The metal catalyst can be also selected frommetal hydrides, metal organic compounds and mixtures thereof, preferablycontaining the metal Me, and preferably supported on and in particulargrafted to a solid support.

The term “metal catalyst supported on and grafted to a solid support” isgenerally understood to mean a metal catalyst comprising a solid supportand at least one metal, preferably the metal Me, which is (chemically)attached to the solid support, in particular by at least a single ormultiple bond, and in particular which is bonded directly to at leastone of the essential elements (or constituents) of the solid support.

The metal, Me, present in the metal catalyst can be at least one metalchosen from the lanthanides, the actinides and the metals from Groups 2to 12, preferably from Groups 3 to 12, in particular from the transitionmetals from Groups 3 to 11, and in particular from Groups 3 to 10, ofthe Periodic Table of the Elements. The metal, Me, can be in particularat least one metal chosen from yttrium, scandium, lanthanum, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, rhenium, iron, ruthenium, cobalt, rhodium, nickel, iridium,palladium, platinum, cerium and neodymium. It can preferably be chosenfrom yttrium, titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, molybdenum, tungsten, ruthenium, rhodium and platinum and moreparticularly from yttrium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, ruthenium, rhodium and platinum.

The metal catalyst can be preferably chosen from metal catalystssupported on and grafted to a solid support, comprising a solid supportand one or more metals, Me, which are identical or different and whichare in particular (chemically) attached to the solid support, especiallyby single or multiple bonds. The metal, Me, atom can, in addition,advantageously be bonded to at least one hydrogen atom and/or to atleast one hydrocarbon radical.

In the case where the metal, Me, grafted to a solid support is bonded toat least one hydrogen atom, the metal catalyst can be chosen fromsupported and grafted metal catalysts comprising a solid support towhich is grafted at least one metal hydride of the metal Me.

In the case where the metal, Me, grafted to a solid support is bonded toat least one hydrocarbon radical, the metal catalyst can be chosen fromsupported and grafted metal catalysts comprising a solid support towhich is grafted at least one organometallic compound of the metal Me.

The metal catalyst can also be advantageously chosen from supported andgrafted metal catalysts comprising a solid support to which are graftedat least two types of metal Me, one in a form (A) of a metal compoundwhere the metal, Me, is bonded to at least one hydrogen atom and/or toat least one hydrocarbon radical, and the other in a form (B) of a metalcompound where the metal, Me, is bonded solely to the solid support andoptionally to at least one other element which is neither a hydrogenatom nor a hydrocarbon radical. In each of the forms (A) and (B), themetal catalyst can comprise one or more different metals, Me. The metalMe present in the form (A) can be identical to or different from thatpresent in the form (B). When the forms (A) and (B) coexist in the metalcatalyst, the degree of oxidation of the metals Me present in the form(A) can be identical to or different from that of the metals Me presentin the form (B).

The solid support can be any solid support, preferably chosen frominorganic solid supports, in particular comprising essentially atoms Mand X which are different from one another and which are generallybonded to one another by single or multiple bonds, so as to form inparticular the molecular structure of the solid support. The term“support comprising essentially atoms M and X” is generally understoodto mean a support which comprises the atoms M and X as predominantconstituents and which can additionally comprise one or more other atomscapable of modifying the structure of the support.

The atom M of the solid support can be at least one of the elementschosen from the lanthanides, the actinides and the elements from Groups2 to 15 of the Periodic Table of the Elements. The atom M of the solidsupport can be identical to or different from the metal Me. The atom Mcan be at least one of the elements chosen in particular from magnesium,titanium, zirconium, cerium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, boron, aluminium, gallium, silicon, germanium,phosphorus and bismuth. The atom M of the solid support is preferably atleast one of the elements chosen from the lanthanides, the actinides andthe elements from Groups 2 to 6 and from Groups 13 to 15 of the PeriodicTable of the Elements, in particular from silicon, aluminium andphosphorus.

The atom X of the solid support, which is different from the atom M, canbe chosen from at least one of the elements from Groups 15 and 16 of thePeriodic Table of the Elements, it being possible for the element to bealone or itself optionally bonded to another atom or to a group ofatoms. In the case where the atom X of the solid support is chosen inparticular from at least one of the elements from Group 15, it canoptionally be bonded to another atom or to a group of atoms chosen, forexample, from a hydrogen atom, a halogen atom, in particular a fluorine,chlorine or bromine atom, a saturated or unsaturated hydrocarbonradical, a hydroxyl group of formula (HO—), a hydrosulphide group offormula (HS—), alkoxide groups, thiolate groups, silylated (or silane)groups or organosilylated (or organosilane) groups. Preferably, the atomX of the solid support is at least one of the elements chosen fromoxygen, sulphur and nitrogen and more particularly from oxygen andsulphur.

The atoms M and X, which generally represent the essential elements ofthe solid support, can in particular be bonded to one another via singleor double bonds. In a preferred alternative form, the solid support canbe chosen from metal oxides, refractory oxides, molecular sieves,sulphated metal oxides, sulphated refractory oxides, metal sulphides,refractory sulphides, sulphided metal oxides, sulphided refractoryoxides and azides.

In particular, the solid support may be chosen from oxides, sulphidesand azides, in particular M, and mixtures of two or three oxides,sulphides and/or azides. More particularly, the solid support can be anoxide, in particular an oxide of M, and can be chosen from simple ormixed oxides, in particular simple or mixed oxides of M, or mixtures ofoxides, in particular mixtures of oxides M. The solid support can, forexample, be chosen from metal oxides, refractory oxides and molecularsieves, in particular from silica, alumina, silicoaluminates, aluminiumsilicates, simple or modified by other metals, zeolites, clays, titaniumoxide, cerium oxide, magnesium oxide, niobium oxide, tantalum oxide andzirconium oxide. The solid support can also be a metal oxide or arefractory oxide, optionally modified by an acid, and can optionallycomprise in particular an atom M bonded to at least two atoms X whichare different from one another, for example the oxygen atom and thesulphur atom. Thus, the solid support can be chosen from sulphated metaloxides or sulphated refractory oxides, for example a sulphated aluminaor a sulphated zirconia. The solid support can also be chosen from metalsulphides, refractory sulphides, sulphided metal oxides and sulphidedrefractory oxides, for example a molybdenum sulphide, a tungstensulphide or a sulphided alumina. The solid support can also be chosenfrom azides, in particular boron azide.

The essential constituents of the solid support are preferably the atomsM and X described above. In addition, the solid support has theadvantage of generally exhibiting, at the surface, atoms X capable offorming part of the coordination sphere of the metal, Me, of the metalcatalyst, in particular when the catalyst is chosen from metal compoundssupported on and grafted to a solid support. Thus, at the surface of thesupport, the atom N which is bonded to at least one metal atom, Me, canadvantageously be additionally bonded to at least one atom M. The bondsbetween X and M and those between X and Me can be single or doublebonds:

In the case of a metal catalyst supported on and grafted to a support,the metal, Me, present in particular in the form (A) can be bonded, onthe one hand, to the support, in particular to at least one atomconstituting the support, preferably the atom X of the support asdescribed above, in particular by a single or double bond, and, on theother hand, to at least one hydrogen atom and/or to at least onehydrocarbon radical, R, in particular by a carbon-metal single, doubleor triple bond. The hydrocarbon radical, R, can be saturated orunsaturated, can have from 1 to 20, preferably from 1 to 10, carbonatoms and can be chosen from alkyl, alkylidene or alkylidyne radicals,in particular C₁ to C₁₀ radicals, preferably C₁ radicals, aryl radicals,in particular C₆ to C₁₀ radicals, and aralkyl, aralkylidene oraralkylidyne radicals, in particular C₇ to C₁₄ radicals.

In the case of a metal catalyst supported on and grafted to a support,the metal, Me, present in particular in the form (A), can be bonded tothe hydrocarbon radical, R, via one or more carbon-metal single, doubleor triple bonds. It can be a matter of a carbon-metal single bond, inparticular of the a type: in this case, the hydrocarbon radical, R, canbe an alkyl radical, in particular a linear or branched radical, forexample a C₁ to C₁₀, preferably C₁, radical, or an aryl radical, forexample the phenyl radical, or an aralkyl radical, for example thebenzyl radical. The term “alkyl radical” is generally understood to meana monovalent aliphatic radical resulting from the removal of a hydrogenatom from the molecule of an alkane or of an alkene or of an alkyne, forexample the methyl, ethyl, propyl, neopentyl, allyl or ethynyl radical.The methyl radical is preferred.

It can also be a matter of a carbon-metal double bond, in particular ofthe π type: in this case, the hydrocarbon radical, R, can be analkylidene radical, in particular a linear or branched radical, forexample C₁ to C₁₀, preferably C₁, radical, or an aralkylidene radical,for example a C₇ to C₁₄ radical. The term “alkylidene radical” isgenerally understood to mean a bivalent aliphatic radical originatingfrom the removal of two hydrogen atoms from the same carbon of themolecule of an alkane or of an alkene or of an alkyne, for example themethylidene, ethylidene, propylidene, neopentylidene or allylideneradical. The metlhylidene radical is preferred. The term “aralkylideneradical” is generally understood to mean a bivalent aliphatic radicaloriginating from the removal of two hydrogen atoms from the same carbonof an alkyl, alkenyl or alkynyl linking unit of an aromatic hydrocarbon.

It can also be a matter of a carbon-metal triple bond: in this case, thehydrocarbon radical, R, can be an alkylidyne radical, in particular alinear or branched radical, for example a C₁ to C₁₀, preferably C₁,radical, or an aralkylidyne radical, for example a C₇ to C₁₄ radical.The term “alkylidyne radical” is generally understood to mean atrivalent aliphatic radical originating from the removal of threehydrogen atoms from the same carbon of the molecule of an alkane or ofan alkene or of an alkyne, for example the metilylidyne, ethylidyne,propylidyne, neopentylidyne or allylidyne radical. The methylidyneradical is preferred. The term “aralkylidyne radical” is generallyunderstood to mean a trivalent aliphatic radical originating from theremoval of three hydrogen atoms from the same carbon of an alkyl,alkenyl or alkynyl linking unit of an aromatic hydrocarbon.

The metal catalyst can advantageously be chosen from metal catalystssupported on and grafted to a solid support comprising the metal, Me,present in both forms (A) and (B). Such a metal catalyst has theadvantage of exhibiting a very high catalytic activity in the reactionfor producing ethane from methane or for the conversion of methane intoethane. The form (A) of the metal catalyst is that described above. Inthe form (B), the metal, Me, is preferably bonded solely to the support,in particular to one or more atoms constituting the essential elementsof the support, in particular to one or more atoms X of the support suchas are described above, for example by single or double bonds.

In the form (B), the metal, Me, can optionally be bonded, in addition tothe support, to at least one other element which is neither a hydrogenatom nor a hydrocarbon radical. The other element bonded to the metal Mecan, for example, be at least one of the elements from Groups 15 to 17of the Periodic Table of the Elements, which element can be alone oritself bonded to at least one hydrogen atom and/or to at least onehydrocarbon radical and/or to at least one silylated (or silane) ororganosilylated (or organosilane) group. In particular, the metal, Me,present in the form (B) can also be bonded, in addition to the support,to at least one atom of the elements chosen from oxygen, sulphur,nitrogen and halogens, in particular fluorine, chlorine or bromine.Thus, for example, the metal, Me, can be bonded, via a single bond, toone or more halogen atoms, in particular fluorine, chlorine or bromine.It can also be bonded, via a double bond, to one or more oxygen orsulphur atoms, in particular in the form of a metal oxide or sulphide.It can also be bonded, via a single bond, to at least one oxygen orsulphur atom itself bonded to a hydrogen atom or to a saturated orunsaturated hydrocarbon radical, in particular a C₁ to C₂₀, preferablyC₁ to C₁₀, radical, for example in the form of a hydroxide, of ahydrosulphide, of an alkoxide or of a thiolate. It can also be bonded,via a single bond, to a silylated or organosilylated group. It can alsobe bonded, via a single bond, to an amido (or amide) group, for exampleof formulae (H₂N —), (HRN—) or (RR′N—) in which R and R′, which areidentical or different, represent saturated or unsaturated hydrocarbonradicals, in particular C₁ to C₂₀, preferably C₁ to C₁₀, radicals orsilylated or organosilylated groups or else can be bonded, via a doublebond, to an imido (or imide) group, for example of formula (HN═), or,via a triple bond, to a nitrido (or azide) group, for example of formula(N═).

It is preferable to use metal catalysts supported on or grafted to asolid support in which the metal, Me, grafted to the support existssimultaneously in both forms (A) and (B), as these catalystsadvantageously exhibit a very high catalytic activity in methanecoupling or homologation reactions. This is in particular the case when,per 100 mol of the metal Me grafted to the support, the metal catalystcomprises:

-   -   (a) from 5 to 95 mol, preferably from 10 to 90 mol, in        particular from 20 to 90 mol, especially from 25 to 90 mol, or        more particularly from 30 to 90 mol, of the metal Me in the form        (A), and    -   (b) from 95 to 5 mol, preferably from 90 to 10 mol, in        particular from 80 to 10 mol, especially from 75 to 10 mol, or        more particularly from 70 to 10 mol, of the metal Me in the form        (B).

The metal catalysts described above can be prepared in various ways. Afirst process for the preparation of a metal catalyst supported on andgrafted to a solid support can comprise the following stages:

-   -   (a) an organometallic precursor (P) comprising the metal Me        bonded to at least one hydrocarbon ligand is grafted to the        solid support, and    -   (b) the solid product resulting from stage (a) is treated with        hydrogen or a reducing agent capable of forming a metal        Me-hydrogen bond, preferably by hydrogenolysis of the        hydrocarbon ligands, at a temperature in particular at most        equal to the temperature T1 at which the catalyst is formed        solely in the form (A) as defined above.

The temperature of stage (b) is chosen in particular so that it is atmost equal to the temperature T1 where only the form (A) of the catalystis formed, that is to say where only the metal hydride is formed. Thetemperature of stage (b) can in particular be chosen within a range from50 to 160° C., preferably from 100 to 150° C. Stage (b) can take placeunder an absolute pressure of 10⁻³ to 10 MPa and for a period of timewhich can range from 1 to 24 hours, preferably from 5 to 20 hours.

A second process for the preparation of a metal catalyst can comprisethe following stages:

-   -   (a) an organometallic precursor (P) comprising the metal Me        bonded to at least one hydrocarbon ligand is grafted to the        solid support, and    -   (b) the solid product resulting from stage (a) is treated with        hydrogen or a reducing agent capable of forming a metal        Me-hydrogen bond, preferably by hydrogenolysis of the        hydrocarbon ligands, at a temperature greater than the        temperature T1 at which the catalyst is formed solely in the        form (A) and less than the temperature T2 at which the catalyst        is formed solely in the form (B), the forms (A) and (B) being        those described above.

The temperature of stage (b) is chosen in particular so that it isgreater than the temperature T1 where only the form (A) is formed. Itcan in particular be at least 10° C., preferably at least 20° C., inparticular at least 30° C. or even at least 50° C. greater than thetemperature T1. It is in addition chosen in particular so that it isless than the temperature T2 where only the form (B) is formed. It canin particular be at least 10° C., preferably at least 20° C., inparticular at least 30° C. or even at least 50° C. less than thetemperature T2. The temperature of stage (b) can, for example, be chosenwithin a range from 165° C. to 450° C., preferably from 170 to 430° C.,in particular from 180 to 390° C., in particular from 190 to 350° C. orfrom 200 to 320° C. Stage (b) can take place under an absolute pressureof 10⁻³ to 10 MPa and for a period of time which can range from 1 to 24hours, preferably from 5 to 20 hours.

A third process for the preparation of a metal catalyst can comprise thefollowing stages:

-   -   (a) an organometallic precursor (P) comprising the metal Me        bonded to at least one hydrocarbon ligand is grafted to the        solid support, then    -   (b) the solid product resulting from stage (a) is treated with        hydrogen or a reducing agent capable of forming a metal        Me-hydrogen bond, preferably by complete hydrogenolysis of the        hydrocarbon ligands, at a temperature in particular at most        equal to the temperature T1 at which the catalyst is formed        solely in the form (A) as defined above, so as to form a metal        hydride in the form (A), and    -   (c) the solid product resulting from stage (b) is heat-treated,        preferably in the presence of hydrogen or of a reducing agent,        at a temperature greater than the temperature of stage (b) and        less than the temperature T2 at which the catalyst is formed        solely in the form (B) as defined above.

Stage (b) of the process can be carried out under the same conditions,in particular of temperature, as those of stage (b) of the firstpreparation process. Stage (c) can be carried out at a temperature,under a pressure and for a period of time equivalent to those describedin stage (b) of the second preparation process.

A fourth process for the preparation of a metal catalyst can comprisethe following stages:

-   -   (a) an organometallic precursor (P) comprising the metal Me        bonded to at least one hydrocarbon ligand is grafted to the        solid support comprising functional groups capable of grafting        the precursor (P) by bringing the precursor (P) into contact        with the solid support so as to graft the precursor (P) to the        support by reaction of (P) with a portion of the functional        groups of the support, preferably from 5 to 95% of the        functional groups of the support, then    -   (b) the solid product resulting from stage (a) is heat-treated,        preferably in the presence of hydrogen or of a reducing agent,        at a temperature equal to or greater than the temperature T2 at        which the catalyst is formed solely in the form (B) as defined        above, then    -   (c) an organometallic precursor (P′), identical to or different        from (P), comprising the metal Me bonded to at least one        hydrocarbon ligand, the metal Me and the ligand being identical        to or different from those of (P), is grafted to the solid        product resulting from stage (b) by bringing the precursor (P′)        into contact with the solid product resulting from stage (b) so        as to graft the precursor (P′) to the support by reaction of        (P′) with the functional groups remaining in the support, and        optionally    -   (d) the solid product resulting from stage (c) is treated with        hydrogen or a reducing agent capable of forming metal        Me-hydrogen bonds, preferably by complete hydrogenolysis of the        hydrocarbon ligands of the grafted precursor (P′), at a        temperature in particular at most equal to the temperature T1 at        which the catalyst is formed solely in the form (A) as defined        above.

Stage (b) of the process can be carried out at a temperature such thatmost, preferably all, of the precursor (P) grafted to the support isconverted to metal compound in the form (B). The temperature duringstage (b) can be chosen within a range from 460° C., preferably from480° C., in particular from 500° C., up to a temperature below thesintering temperature of the support. Stage (d) is optional and can becarried out at a temperature equivalent to that of stage (b) of thefirst preparation process.

A fifth process for the preparation of a metal catalyst can comprise thefollowing stages:

-   -   (a) an organometallic precursor is grafted to the solid support        under the same conditions as in stage (a) of the preceding        preparation process, then    -   (b) the solid product resulting from stage (a) is treated under        the same conditions as in stage (b) of the preceding preparation        process, then    -   (c) the solid product resulting from stage (b) is brought into        contact with at least one compound Y capable of reacting with        the metal Me of the form (A) and/or (B), prepared above, the        contacting operation preferably being followed by removal of the        unreacted compound Y and/or by a heat treatment at a temperature        below the sintering temperature of the support, then    -   (d) an organometallic precursor (P′), identical to or different        from (P), comprising the metal Me bonded to at least one        hydrocarbon ligand, the metal Me and the ligand being identical        [lacuna] or different from those of (P), is grafted to the solid        product resulting from stage (c) by bringing the precursor (P′)        into contact with the product resulting from stage (c) so as to        graft the precursor (P′) to the support by reaction of (P′) with        the functional groups remaining in the support, and optionally    -   (e) the solid product resulting from stage (d) is treated with        hydrogen or a reducing agent capable of forming metal        Me-hydrogen bonds, preferably by complete hydrogenolysis of the        hydrocarbon ligands of the grafted precursor (P′), at a        temperature in particular at most equal to the temperature T1 at        which the catalyst is formed solely in the form (A) as defined        above.

Stage (b) of the process can be carried out at a temperature equivalentto that of stage (b) of the fourth preparation process. In stage (c),the compound Y can be chosen from molecular oxygen, water, hydrogensulphide, ammonia, an alcohol, in particular a C₁ to C₂₀, preferably C₁to C₁₀, alcohol, a thiol, in particular a C₁ to C₂₀, preferably C₁ toC₁₀, thiol, a primary or secondary C₁ to C₁₀, preferably C₁ to C₁₀,amine, a molecular halogen, in particular molecular fluorine, chlorineor bromine, and a hydrogen halide, for example of formula HF, HCl orHBr. The heat treatment optionally carried out at the end of stage (c)can be carried out at a temperature ranging from 25 to 500° C. Stage (e)is optional and can be carried out at a temperature equivalent to thatof stage (b) of the first preparation process.

In the processes for the preparation of a supported and grafted metalcatalyst such as are described above, the operation of grafting to asolid support employs at least one organometallic precursor (P) or (P′)comprising the metal Me bonded to at least one hydrocarbon ligand. Theprecursor can correspond to the general formula:MeR′_(a)  (2)in which Me has the same definition as above, R′ represents one or moreidentical or different and saturated or unsaturated hydrocarbon ligands,in particular aliphatic or alicyclic ligands, in particular C₁ to C₂₀,preferably C₁ to C₁₀, ligands, for example having the same definition asthat given above for the hydrocarbon radical. R, of the metal catalyst,and a is an integer equal to the degree of oxidation of the metal Me.The radical R′ can be chosen from alkyl, alkylidene, alkylidyne, aryl,aralkyl, aralkylidene and aralkylidyne radicals. The metal Me can bebonded to one or more carbons of the hydrocarbonaceous ligands, R′, inparticular via carbon-metal single, double or triple bonds, such asthose connecting the metal Me to the hydrocarbon radical, R, in thecatalyst.

In the processes for the preparation of a supported and grafted metalcatalyst such as are described above, the solid support is preferablysubjected beforehand to a dehydration and/or dehydroxylation heattreatment, in particular at a temperature below the sinteringtemperature of the support, preferably at a temperature ranging from 200to 1000° C., preferably from 300 to 800° C., for a period of time whichcan range from 1 to 48 hours, preferably from 5 to 24 hours. Thetemperature and the duration can be chosen so as to create and/or toallow to remain, in the support and at predetermined concentrations,functional groups capable of grafting by reaction the precursor (P) or(P′). Mention may be made, among functional groups known for thesupports, of groups of formulae XH in which H represents a hydrogen atomand X corresponds to the same definition as given above for the supportand in particular can represent an atom chosen from oxygen, sulphur andnitrogen. The most well-known functional group is the hydroxyl group.

The grafting operation can generally be carried out by sublimation or bybringing the precursor into contact in a liquid medium or in solution.In the case of a sublimation, the precursor, used in the solid state,can be heated under vacuum and the temperature and pressure conditionswhich provide for its sublimation and its migration in the vapour stateonto the support. The sublimation can be carried out at a temperatureranging from 20 to 300° C., in particular from 50 to 150° C., undervacuum.

A grafting can also be carried out by carrying out the contactingoperation in a liquid medium or solution. In this case, the precursorcan be dissolved in an organic solvent, such as pentane or ethyl ether,so as to form a homogeneous solution, and the support can subsequentlybe suspended in the solution comprising the precursor or by any othermethod which provides for contact between the support and the precursor.The contacting operation can be carried out at ambient temperature (20°C.) or more generally at a temperature ranging from −80° C. to +150° C.,under an inert atmosphere, such as nitrogen. If only a portion of theprecursor has become attached to the support, the excess can be removedby washing or reverse sublimation.

The process of the present invention makes it possible to carry out aconversion of methane by a methane coupling or homologation reactionwith an extremely high selectivity by weight for ethane with respect tocarbon-containing products formed. The advantage of this process is thatof being able to rapidly recover and isolate the ethane produced, simplyby separating the ethane from unreacted methane and hydrogen formed. Theethane, thus recovered and isolated, can be employed in processes forenhancing, the value of ethane, for example in dehydrogenation,catalytic cracking or thermal cracking processes, optionally in thepresence of steam, so as to selectively manufacture in particularolefins, such as ethylene.

The following examples illustrate the present invention.

EXAMPLE 1 Preparation of a Tantalum Catalyst

A tantalum catalyst supported on and grafted to silica was prepared inthe following way.

In a first step, 5 g of a silica dehydrated and treated at 500° C.beforehand and then 20 ml of an n-pentane solution comprising 800 mg(1.72 millimol of tantalum) of tris(neopentyl)neopentylidenetantalum,used as precursor and corresponding to the general formula:Ta[—CH₂—C(CH₃)₃]₃[═CH—C(CH₃)₃]  (3)were introduced under an argon atmosphere into a glass reactor. Theprecursor was grafted at 25° C. to the silica, in particular by reactionwith the hydroxyl groups of the silica. The excess precursor which hadnot reacted was removed by washing with n-pentane. The resulting solidcompound, which constituted the organometallic compound grafted to thesilica and which corresponded to the general formula:(Si—O)_(1.35)Ta[═CH—C(CH₃)₃][—CH₂—C(CH₃)₃]_(1.65)  (4)was then dried under vacuum.

In a second stage, the tantalum compound, thus supported on and graftedto the silica, was subsequently treated under an atmosphere of 80 kPa ofhydrogen at a temperature of 250° C. for 15 hours. By hydrogenolysis ofthe neopentyl and neopentylidene ligands, a tantalum catalyst supportedon and grafted to silica was formed which, per 100 parts by moles oftantalum, comprised:

-   -   72 parts by moles of a tantalum hydride grafted to the silica in        the form (A) corresponding to the general formula:        [(silica support)-Si—O]₂—Ta—H  (5)    -   and    -   28 parts by moles of a tantalum compound grafted to the silica        in the form (B) corresponding to the general formula:        [(silica support)-Si—O]₃—Ta  (6)

EXAMPLE 2 Methane Coupling Reaction

Conversion of methane by a coupling reaction, converting methaneessentially to ethane, was carried out in the following way.

Methane was passed continuously according to a flow rate of 35 ml/min(i.e. 1.33×10⁻⁴ mol of methane per minute), under a total absolutepressure of 5 MPa, through a reactor with a capacity of 5 ml which washeated at 250° C. and which comprised 300 mg of the tantalum catalystprepared in Example 1 (59.3 micromol of tantalum in the form (A)).

Bringing methane into contact with the tantalum catalyst resulted in theformation of ethane and hydrogen, according to the methane couplingreaction written according to the following equation:2CH₄→C₂H₆+H₂  (1)

It was observed that the conversion of the methane exhibited aselectivity by weight for ethane with respect to carbon-containingproducts formed of greater than 99.9%. In particular, no detectabletrace was found of alkene or alkyne formed, such as ethylene oracetylene, or of aromatic compounds, such as benzene, of carbon monoxideor of carbon dioxide.

During the reaction, the instantaneous concentrations of hydrogen,methane, ethane and carbon-containing products formed in the conversionwere regularly measured, and the instantaneous molar ratios of ethaneformed and hydrogen formed with respect to the tantalum of the catalyst(respectively [C₂]_(ins)/[Ta] and [H₂]ins/[Ta]), the cumulative molarratios of ethane formed and hydrogen formed with respect to the tantalumof the catalyst (respectively [C₂]_(cum)/[Ta] and [H₂]_(cum)/[Ta]) andthe instantaneous percentage of conversion of the methane (% C₁ conv)were calculated. The results of these measurements and calculations werecollated in Table 1. TABLE 1 Time [C₂]_(ins)/ [H₂]_(ins)/ % [C₂]_(cum)/[H₂]_(cum)/ (min) [Ta] [Ta] C₁ conv. [Ta] [Ta] 1 500 8.0 × 10⁻⁵ 1.5 ×10⁻⁴ 0.028 0.25 0.75 3 000 7.5 × 10⁻⁵ 1.05 × 10⁻⁴  0.026 0.55 1.3 6 0006.0 × 10⁻⁵ 7.5 × 10⁻⁵ 0.021 1.1 2.05 8 500 5.5 × 10⁻⁵ 6.0 × 10⁻⁵ 0.0181.5 2.5

In addition, Table 1 shows that the number of moles of methane which hadreacted per mole of tantalum, after reacting for 8500 minutes, was equalto approximately 3.

EXAMPLE 3 Methane Coupling Reaction

Conversion of methane to ethane, by a coupling reaction of the latter,was carried out exactly as in Example 2, except that the reactor washeated at different temperatures (instead of 250° C.), namely at 300°C., 375° C. and 475° C. respectively in Runs A, B and C.

In each run, the coupling reaction was carried out during 6500 minutes,and at that time, the instantaneous concentrations of methane, ethaneand carbon-containing products formed in the conversion were measured,and the instantaneous percentage of conversion of the methane (% C₁conv), the number of moles of methane having reacting per mole oftantalum (C₁ reacted/Ta) and the selectivity by weight for ethane (C₂H₆selectivity) were calculated. The results of these measurements andcalculations were collated in Table 2. TABLE 2 C₂H₆ Temperatureselectivity Run (° C.) % C₁ conv C₁ reacted/Ta (%) A 300 0.054 6.2 99.7B 375 0.102 14.3 99.0 C 475 0.227 33 96.1

Table 2 shows that the number of moles of methane having reacted permole of tantalum had greatly increased by a factor of about 5.3 between300° C. and 475° C., while the slectivity by weight for ethane had onlyslightly decreased from 99.7 to 96.1%.

EXAMPLE 4 Preparation of a Tungsten Catalyst

A tungsten catalyst supported on and grafted to silica was preparedexactly as in Example 1, except that, in the first stage, instead ofusing a solution of tris(neopentyl)neopentylidenetantalum in n-pentane,use was made of a solution of tris(neopentyl)neopentylidynetungsten inn-pentane, corresponding to the general formula:W[—CH₂—C(CH₃)₃]₃[═C—C(CH₃)₃]  (7)and that, in the second stage, instead of carrying out thehydrogenolysis at 250° C., it was carried out at 150° C. A tungstencatalyst supported on silica was thus obtained essentially in the form(A) of a tungsten hydride.

EXAMPLE 5 Methane Couplings Reaction

Conversion of methane to ethane, by a coupling reaction of the latter,was carried out as in Example 2, except that, instead of using thetantalum catalyst prepared in Example 1, the tungsten catalyst preparedabove in Example 4 was used.

Under these conditions, it was observed that the methane couplingreaction resulted in the formation of ethane and hydrogen with aselectivity by weight for ethane with respect to carbon-containingproducts formed of greater than 99.9%. In particular, no detectabletrace was found of alkene or alkyne formed, such as ethylene oracetylene, or of aromatic compound, such as benzene, of carbon monoxideor of carbon dioxide.

1. Process for producing ethane, characterized in that it comprisesbringing methane into contact with a metal catalyst selected from metalhydrides, metal organic compounds and mixtures thereof.
 2. Processaccording to claim 1, characterized in that the metal catalyst comprisesat least one metal, Me, chosen from the lanthanides, the actinides andthe metals from Groups 2 to 12, preferably 3 to 12, of the PeriodicTable of the Elements.
 3. Process for the conversion of methane tocarbon-containing products, characterized in that methane is broughtinto contact with a metal catalyst comprising at least one metal, Me,chosen from the lanthanides, the actinides and the metals from Groups 2to 12, preferably 3 to 12, of the Periodic Table of the Elements, so asto produce ethane in a proportion of at least 65% by weight with respectto carbon-containing products formed in the process.
 4. Processaccording to claim 3, characterized in that ethane is produced in aproportion of at least 70%, preferably of at least 80%, in particular ofat least 90%, especially of at least 95%, more especially of at least98% or 99%, by weight with respect to carbon-containing products formedin the process.
 5. Process according to claim 3 or 4, characterized inthat the metal catalyst is chosen from metal hydrides, metal organiccompounds and mixtures thereof.
 6. Process according to any one ofclaims 1 to 5, characterized in that it is carried out under conditionsinvolving a non-oxidative coupling of methane.
 7. Process according toany one of claims 1 to 6, characterized in that it comprises bringingmethane into contact with the metal catalyst in the presence of one ormore other initial alkanes, preferably C₂ to C₃₀ alkanes.
 8. Processaccording to any one of claims 1 to 7, characterized in that it is asingle-stage process.
 9. Process according to any one of claims 1 to 8,characterized in that it is carried out with operating conditionsmaintained substantially constant, preferably continuously, during theethane production.
 10. Process according to any one of claims 1 to 9,characterized in that it is carried out under a total absolute pressureranging from 10⁻³ to 100 MPa, preferably from 0.1 to 50 MPa, inparticular from 0.1 to 30 MPa or from 0.1 to 20 MPa, especially from 0.1to 10 MPa.
 11. Process according to any one of claims 1 to 10,characterized in that it is carried out at a temperature ranging from−30° C. to +800° C., preferably from 0 to 600° C., in particular from 20to 500° C., especially from 50 to 450° C.
 12. Process according to anyone of claims 1 to 11, characterized in that it is carried out in thepresence of one or more inert agents, in particular liquid or gaseousinert agents, especially in the presence of one or more inert gases. 13.Process according to any one of claims 1 to 12, characterized in thatthe metal catalyst is chosen from metal catalysts supported on andpreferably grafted to a solid support.
 14. Process according to claim13, characterized in that the solid support is chosen from inorganicsolid supports, preferably from metal oxides, refractory oxides,molecular sieves, sulphated metal oxides, sulphated refractory oxides,metal sulphides, refractory sulphides, sulphided metal oxides, sulphidedrefractory oxides and azides.
 15. Process according to any one of claims1 to 14, characterized in that the metal, Me, of the metal catalyst isat least one metal chosen from yttrium, scandium, lanthanum, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,tungsten, rhenium, iron, ruthenium, cobalt, rhodium, iridium, nickel,palladium, platinum, cerium and neodymium.
 16. Process according toclaim 15, characterized in that the metal, Me, is at least one metalchosen from yttrium, titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, ruthenium, rhodium andplatinum.
 17. Process according to claim 15, characterized in that themetal, Me, is at least one metal chosen from yttrium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, ruthenium, rhodium andplatinum.
 18. Process according to any one of claims 1 to 17,characterized in that it is carried out in gas phase, in particular in afluidized bed reactor and/or a reactor with a mechanically stirred bed,or a stationary bed reactor or circulating bed reactor.
 19. Processaccording to claim 18, characterized in that the metal catalyst is usedin a solid form essentially forming the bed of the reactor.